An evil scientist has engineered a plague that would kill anyone with an xy chromosome pair. This plague has systemicatically spread throughout the planet, killing all males within the population. All men on the planet, including those in utero, have been wiped out.

Many have blamed the plague on God punishing humanity for their sins, while others look for some logical cause. In either case, economic collapse, instability, starvation, and emotional pain and suffering has resulted around the world. One man has survived due to his genes, and is resistant to the plague that has wiped out other males.

After a number of years, an organization has risen to power and becomes a source of stablility in the world. It has located and captured the last living male and plans to use him to save the human race. This specimen is about 25 years of age in reasonably healthy condition. After putting him into artificially induced coma, they have used technology to repopulate the species through in - vitro fertilization.

Can the population survive long term when all offspring originate from one father? And after taking into account miscarriages and stillbirths, how many offspring can this individual have?

$\begingroup$This is kinda the plot for Y: The Last Man.$\endgroup$
– RenanDec 9 '18 at 20:16

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$\begingroup$better hope that male does not have any defects in his Y chromosome.$\endgroup$
– JohnDec 9 '18 at 22:15

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$\begingroup$Incognito, I have GOT to ask if your mispelling of @chaslyfromUK's moniker was intentional or simply freudian.$\endgroup$
– JBHDec 9 '18 at 23:24

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$\begingroup$@JBH - lol! I've been married twice! No chance of that.$\endgroup$
– chasly from UKDec 9 '18 at 23:39

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$\begingroup$Loophole alert: If that humanity is anything like present-day Earth, there will be a lot of sperm from diverse donors frozen in sperm banks. If civilization did not collapse in even a few mid-sized cities, they will remain frozen through the apocalypse. The women can repopulate without that surviving male.$\endgroup$
– Emilio M BumacharDec 10 '18 at 17:17

6 Answers
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More like one man and 3.9 billion women? If your one man has good genes, has no problem down the belt, is not too old or going to die soon or make stupid decisions, your population is safe from extinction with close to zero optimization and control.

Your man may not be doctor or biology specialist, but there will be a lot of them in women pool. So there is no reason to think they won't collect gene material from the man and millions of women to mass produce population and make reserves for future.

And descendants of the man and their genes can be used to do the same. Even his grandchildren will make it before younger part of surviving women hit menopause.

After that you can even use stored eggs even after that. Extra control to prevent inbreeding and you are golden.

About amount of children he can have, with collection of semen for artificial use. That would depend on quality of his semen, how long they can go on or for how long he will live or stay fertile and how good your specialists are.Good way to improve your numbers is to make the man live on good diet with regular gym sessions. Making babies via medicine is more effective.

You would expect around 3-8 pregnancies each month. More advanced and invasive methods in good hands can make number go up an order. And that number will go even higher with advances in the most needed field for your population and add on top ways to do so with no need of said man.

First generation males after the plague can still reproduce with any of the women from before the plague, and there would be no overlap in genes. You could get a few generations before the youngest woman pre-plague hits menopause. At third generation, you have kids with only ~12% of the DNA of the single surviving man, at fourth generation, it's down to just above ~6%.

That doesn't even consider freezing the eggs of the existing women, which could let you do even more generations. There's also gene editing, because those ethics be damned when world collapses.

You have missed a trick. It is better to impregnate a large number of women from this one man but then impregnate other women from his sons when they are able to become fathers. They will produce a mix of sons and daughters. Then avoid incest as much as much as possible to produce diverse grandchildren.

When 100 years have gone by the original man dies together with most of the generation of only women. Then you are once again back to a 50/50 population. At this point you can introduce laws against incest. Currently it is considered acceptable in most societies to allow marriage between sufficiently distant cousins. That would be easy to arrange.

The genetic diversity would be available in the original pool of women. Their sons would inherit a more and more dilute genetic imprint from their 'founding father'. Of course the XY's are back in one generation.

There's some maths to be done but I'm pretty sure it would all happen rather quickly. Maybe in a hundred years. Remember that generations overlap.

A man can produce millions of swimmers everyday. IVF methods uses only a fraction of those, because many are of low quality. But you can safely assume about 100-1000 of women can be impregnated per day. Half of those children will be daughters who are sadly out of luck, because all the boys are their brothers or half brothers. But you can still assume about 100 boys will be born every day for next couple of years, after the initial setup and first pregnancies, so about a year. That means you hit 100'000 in couple of years already.

When the first generation of boys reach biological maturity there will still be plenty of fertile women who haven't been included in the original program. Some of them will be only few years older than the young men if they were children when the original plague happened. So they can be impregnated by the new donors through the same program, or maybe also in more natural methods. I imagine these men will be extremely valued and be basically running their private harems. You will probably hit the capacity of fertilization cliniques at this point so it makes sense to allow actual social polygamy (as opposed to biological "polygamy" of gene-banks).

In the second generation you have millions of boys and girls, who only 30-40 years after the plague are able to start normal families, providing they choose carefully. Basically marriage between full first-cousins should still be banned but there is plenty of half-cousins (different grandmothers, one same grandfather - the original Y-man) to choose from. While this bears some risks, it's tolerable, especially in the context. There are some cultures in our world where marriages between first cousins are the norm, sometimes even between closer relatives (uncle+niece), and this cause disproportional high occurrence of genetic diseases it's better than extinction.

In conclusion, if you have an artificial insemination technology you can safely rebuild the world population before the original women die of age. You still have scientists and technicians alive who remember the old world, so you won't even experience serious discontinuity in cultural or scientific development.

It is very unlikely that many women would have the same gene responsible for the resistance (based on this man's example: one in 8000 M). I would buy for 5 or 10 to exists at the moment that could breed. One hundred would seem ludicrous or proof of an external influence.

If the gen is dominant you wouldn´t have a problem, but if it's recessive then you have a problem. The question is, if the father is dead it's because it's recessive.

You would have to inseminate all women of the world in order to get a few resistant new males. You would need optimistically 11 000 years to achieve it if the man lived so long but you only have his lifespam to obtain the sperm. You would have 0,36% of chances to do if he lives 40 more years and there is only one such woman. So you'd need to inseminate her offprings to get the coupled genes. But I'm not sure this is certain, because epigenetics don't always work like that, it might be impossible.
Evidently you might have the mother (alive and with some eggs)...

Unless... we cloned him. That would be another mess because genetic from clones tends to be very deficient. But in theory, since the man cells needed for cloning can be preserved indefinitely in cryogenic tanks, sometime in a distant future one of hundreds of clones would be the new father of humanity.

$\begingroup$ah, very good point. I think most responders, like me, assumed the plague is over. if the pathogen is still out there the future is much more bleak. Dominant gene gives immunity but only to the carriers, which mean many children will be born without it for many generations. Like color-blindness, the susceptibility to the plague can be passed by women on to their sons.$\endgroup$
– Milo BemDec 11 '18 at 10:42

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$\begingroup$If the gene is recessive it means the man either has two copies or the gene is located on X-chromosome. In either case the humanity is pretty much doomed, he could only produce a small number of resistant but heavily inbred (grand)sons by impregnating his own daughers.$\endgroup$
– Milo BemDec 11 '18 at 10:44

Women don't actually need men to create children, I believe. There is something in the bone marrow that can impregnate another woman, and the children would all be female, so realistically, if he could father many many children and the women could create more women, the planet could eventually, very slowly, repopulate. He wouldn't even be entirely necessary, he would just be there to create males.

$\begingroup$Hello! I'm not aware of anything in the bone marrow that will impregnate a female. Since the tags for this query are science-based, biology and genetics, the burden of supporting evidence for the claim falls upon you. In other words, you will need to support your assertion. Also, welcome to Worldbuilding: please take a moment to review the help center and tour so you can learn what this forum is about and how to offer top quality answers and also what kinds of questions are allowed here.$\endgroup$
– elemtilasDec 10 '18 at 5:33

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$\begingroup$Well, bone marrow contains stem cells which may be coaxed into becoming spermatogonial stem cells, which may then be coaxed into forming primary and secondary spermatocytes which may then be coaxed into forming spermatozoa which then need to mature ... but, you should put that in your answer, preferably with references, and, if at all possible, show that this process does not involve any genes present on the Y-chromosome that cannot be administered in vitro.$\endgroup$
– GretchenVDec 10 '18 at 9:28

$\begingroup$Parathogenesis? Uses the existing egg without fertilization which means it is a clone.$\endgroup$
– RobusDec 10 '18 at 17:17

$\begingroup$you could take a sperm and remove the DNA and fill it with the DNA of an egg and afterwards do "normal" in-vitro. That would always lead to girls, of cause.$\endgroup$
– Julian EgnerDec 11 '18 at 14:15